The development of electrical machines for use in high temperature environments places significant demands on components associated with the machines including a requirement for stability of the materials from which the components are constructed. Environments requiring stability of insulating coatings at high temperature include those associated with nuclear reactors and next generation aircraft motors and generators.
In addition to heat from an environment in which a component is situated, a component may be subject to heat due to other factors such as an electrical current carried by a conductor as well as other stresses. For example, electrical wires used to form windings for motors and generators are subject to particularly harsh thermal and mechanical conditions. The integrity of coatings of such wires is critical to continued successful operation of the motor or generator.
A major barrier restricting the operating temperature of electrical machines is the limited thermal stability of insulation materials applied to the wire from which windings of the machines are formed, as well as the limited stability of insulation materials applied to the windings themselves. Breakdown of insulation materials can occur at excessively high temperatures, or following prolonged exposure of the insulation materials to high temperatures.
The term “high temperature wire” is conventionally used to describe wire insulated with a polymer such as polyimide or polytetrafluoroethylene with a service temperature limited to about 250° C. However, new applications such as those described above may require insulation material that can withstand temperatures of 500° C. or higher. Such temperatures generally preclude the possibility of using organic polymers and therefore the use of inorganic materials has been explored.
U.S. Pat. No. 5,468,557 discloses a method for manufacturing stainless steel clad copper wire coated with an insulator which may be alumina, silica or aluminium nitrite. The insulator is applied to the conductor by means of plasma CVD ion plating. The insulator thickness is limited to around 3 to 4 μm due to brittleness of the insulator material, which limits the breakdown voltage to around 400V.
U.S. Pat. No. 6,876,734 discloses a conductor coated with an insulator composition containing a zirconium compound and a silicon compound which is itself coated with a bonding agent comprising polyamide or polyimide. The high proportion of organic material of the insulator composition imparts good mechanical properties but limits the operating temperature to a maximum of 420° C.
U.S. Pat. No. 5,139,820, EP 0292780 and EP 0460238 disclose conducting wires coated with an insulator formed from alkoxide precursors such as tetraethoxysilane produced by a sol-gel method. U.S. Pat. No. 5,139,820 discloses adding at least one thermoplastic polymer or monomer to the mixture to make the gel extrudable.